Rockets, missiles, and other rocket-propelled vehicles that travel through and outside the earth's atmosphere can experience severe operating conditions. Temperature extremes are one kind of harsh condition that vehicle design and component design must address. Temperatures in space approach absolute zero. However, certain vehicle parts, including for example, valves and nozzle bodies, which for instance are often located in the vehicle's propulsion or attitude control systems, can be subject to hot gas effluent that reaches extremely high temperatures. The temperature in rocket exhaust, for example, can reach levels greater that 5000° F. Pressures in exhaust bodies can also exceed 1000 psi.
Thus material selection is an important criteria in designing valve and nozzle components in rocket applications. Over the years, various materials have been identified which to some extent withstand the temperatures and stresses experienced by hot gas valves and nozzles. Carbon, and particularly the graphite form of carbon, for example, possesses physical properties which make it a useful construction material. Graphite demonstrates high strength and dimensional stability at elevated temperatures. Other carbon structures, such as carbon fibers in a carbon matrix, carbon-carbon, also have excellent high temperature strength. These carbon materials can be used at elevated temperatures where other refractory materials lose their practical strength.
Disadvantageously, carbon and carbon composites are susceptible to corrosion, oxidation, and erosion when exposed to oxidizing or corrosive environments. The environment in rocket exhaust gases is one kind of hostile environment that can lead to the breakdown of carbon structures. Thus it has become known in the art to use a protective coating over the surface of carbon materials exposed to rocket exhaust.
Rhenium is one metal that has been shown to successfully protect carbon materials from erosive and corrosive environments. Various methods have been practiced to form a rhenium layer over carbon-type substrates. Some known methods include electroplating and chemical vapor deposition. Rhenium metal coatings have been used in particular on carbon substrates to protect from the erosion effects of hot high speed gas flow from rocket combustions. This technology is used on rocket nozzles and thrust vector control (TVC) valve parts that require little or no dimensional change during the exposure to hot flowing gases from, for example, solid rocket motors.
The prior art methods of providing protective rhenium coatings have nevertheless experienced limitations and drawbacks. One problem that has been encountered is the loss of adhesion between the rhenium coating and the carbon substrate. Operating conditions that include thermal shock and high temperature and pressure can weaken the adhesion of the coating. As a result coverage by the rhenium coatings is sometimes lost. Rhenium coatings sometimes flake off thereby exposing the underlying carbon substrate. When this happens, the carbon substrate can be significantly and even completely destroyed by rocket exhaust. The loss of rhenium coating thus results in a reduced performance of rocket nozzle or complete loss of valve function in the TVC system.
A source of the difficulty encountered in rhenium/carbon systems is that rhenium and carbon interact. Elemental rhenium has a very high melting point. When exposed to carbon at very high temperatures, however, rhenium and carbon may interact such that carbon decreases the rhenium melting point. The lowered melting point can lead to liquefaction of the rhenium coating at the carbon/rhenium contact interface. The liquefaction thus leads to loss of adhesion and flaking of the rhenium coating.
Hence there is a need for an improved method to bond rhenium to carbon substrates and particularly carbon substrates found in rocket nozzles and TVC valves. There is a need for an improved method that provides strong adhesion between a carbon substrate and a rhenium coating. Moreover there is a need for an improved bonding method that is capable of withstanding extremely high temperatures and pressures such as those associated with rocketry environments. The present invention addresses one or more of these needs.